1. Field of the Invention
The present invention relates to a radiation imaging apparatus and radiation imaging system that are suitable for use in medical diagnostics and industrial nondestructive inspection. As used herein, the term “radiation” refers to electromagnetic waves including X-rays and γ rays and radiation rays including α rays and β rays.
2. Description of the Related Art
In recent years, digital radiation imaging apparatuses including a conversion element that converts a radiation ray or a light ray into electric charge and are made from a non-single crystal semiconductor, such as amorphous silicon or amorphous selenium, have been commercially available. In addition, digital radiation imaging apparatuses including a photoelectric conversion element that converts a radiation ray or a light ray to electric charge and which are made from a single crystal semiconductor, such as a charged-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS), have been commercially available. Some radiation imaging apparatuses employ an indirect method. In the indirect method, a conversion element includes a phosphor that converts a radiation ray to a visible light ray and a photoelectric conversion element that converts the visible light ray to electric charge. The photoelectric conversion element is primarily made of amorphous silicon. In contrast, some radiation imaging apparatuses employ a direct method. In the direct method, a conversion element is primarily made of amorphous selenium, and directly converts a radiation light ray to electric charge. In both methods, radiation imaging apparatuses having large dimensions and a thin shape can be achieved. Accordingly, such radiation imaging apparatuses are also referred to as “flat panel detectors (FDPs)”. Such radiation imaging apparatuses can significantly reduce the period of time from when an image is captured until an observer can view the image.
These radiation imaging apparatuses may be influenced by the time elapsed from when the apparatuses are powered on and a bias is provided to the conversion element, a period of time during an image capturing operation, and an amount of radiation reaching the apparatuses. Accordingly, the characteristics of the radiation imaging apparatuses may vary, and therefore, an image signal acquired by the radiation imaging apparatuses may vary. For example, the dangling bond or defects of the conversion element functions as a trap level, so that a dark current varies. Alternatively, image ghosting (a lag) may occur or vary due to the influence of past radiation or past light irradiation. In addition, due to at least one of the above, the sensitivity of the conversion element that defines an input and output characteristic between the input of radiation or light and the output of electric charge may vary.
Accordingly, for example, U.S. Pat. Nos. 6,965,111 and 5,905,772 describe a technology in which, before a radiation ray or a light ray used for acquiring a subject image is emitted to a radiation imaging apparatus, a light ray not used for acquiring the subject image is emitted from another light source. In this way, variations in the characteristics of the apparatus and variation in an acquired image signal can be reduced.